Diatoms from Soil Cores

 

Diatoms on a Slide from a Soil Core

Background:

                  Diatoms are single celled organisms that have shells composed of silica. These incredibly diverse tiny organisms can be found in both freshwater and marine ecosystems. They are often studied as important indicators of past ecological conditions, because when the diatom dies, the silica shells remain in the sediment.  Because they remain in the sediment for long periods of time, the composition of diatoms found at a specific location in a soil core can tell researchers what conditions were like when that layer was deposited. Diatoms can indicate water salinity, temperature, and pH, as well as show nutrient and pollution levels in a system. For example, imagine a system that experienced no pollution until humans developed the surrounding area. The diatom composition before and after human development would differ. You would expect to see more pollution tolerant diatoms in the soil after development, and fewer pollution sensitive diatom species.

 

Taking a Soil Core at Beach City

Extracting a Core

                   Sediment core extraction required several pieces of equipment. Two boats were used to navigate the body of water (lake) in order to find a desired location for an extraction. Once a desired location was found, the boat remained stationary for a minute to ensure no sediment agitation occurred before taking a sample. Then, the core tube was slowly lowered into the water and forcefully pushed into the sediment, reaching a desired depth. The extraction of the core was performed by careful lifting of the sediment core tube, followed by a quick placement of a rubber stopper at the end of the tube. Finally, both ends of the core tube were plugged and taped to insure preservation of the sample. While, performing a sediment core extraction, it was observed that the bottom of the core was fairly dark due to anoxic conditions that are associated with deeper sediment layers. The top however, observed a brown-dark greenish color, indicating higher levels of oxygen. While extruding the sediment core, the pH of water was tested using a pH meter, which indicated a fairly neutral and acceptable value within the range of 6.5 to 8.  

  

Removing the Bottom Stopper and Inserting Plunger

 Creating Samples
                  Samples from a sediment core are important in studying the history of a desired body of water. They are great indicator of historical biodiversity which can be used to recreate past environments conditions. This information can be compared to the present biodiversity and further analyzed in order to grasp a better understanding of the changing environmental conditions such as the pH, as well as nutrient and pollution concentrations within the body of water. 

Extruding the Soil Core

                  In order to create samples from the extruded core, whirlpac bags were obtained and labeled to ensure accurate tracking of samples. Then, the water layer present inside the sediment core tube was expelled using a plunger- like tool that pushed the sediment from the bottom to the top. A specially designed tray was fitted on top of the sediment core which was used to concentrate the sediment samples into 1 cm intervals. The intervals were collected (by scraping the top) and stored inside designated whirlpac bags. All of the samples collected were then further analyzed by preparing microscopic slides in order to observe diatom biodiversity. 

This Plate Helps Measure Out 1cm of soil

Samples are Scraped into Whirlpak Bags for Sampling

 Preparing Slides

                  To analyze the diatoms in the sample, they must be spread out onto microscope slides. The first step in slide preparation is to remove as much of the organic matter and soil from your samples. This can be done by boiling a small about of sample in nitric acid, to break down organic compounds. After the samples are boiled, they are rinsed with water several times to remove traces of acid. The finished diatom mixture is cloudy and will need to be diluted farther before it can be made into slides. It is recommended that you make three separate dilutions of each sample while making slides. 

Boiling Samples to Purify Them

A Purified Sample: Ready for Dilution and Slides

                  The slides themselves are made by placing the sample onto a clean cover slip.  The sample should cover the entire cover slip and form a mound in the middle. These will need to sit for at least twenty-four hours, as they dry.  Once dry, the cover slips can be permanently mounted to a clean microscope slide.  Attaching the cover slips permanently allows researchers to preserve samples for later analysis.

Alison Demonstrates How to Correctly Make A Slide

Make sure to get a mound of water in the middle of your cover slip. It shouldn't spill onto other cover slips.

 Some of the diatoms found include:

Cymatopleura

Stauroneis

Stephanodiscus

Navicula

Pinnularia

Cocconeis

Cymbella

Aulacosara

Ctenophera

Puncticulata

Nitzschia

Diatoma

Rhopalodia

Eunotia

Frustulia

Asterionella

Platessa

Surirella

Gyrosigma

Enopnema

Pleurosira

Cymbella Seen Under the Microscope